Properly-functioning kidneys remove wastes and excess fluid from the blood. Blood tests can be used to show whether the kidneys are failing to remove wastes, and urine tests can show how quickly body wastes are being removed and whether the kidneys are leaking abnormal amounts of protein.
Serum creatinine. Creatinine is a waste product that comes from meat protein in the diet and from the normal wear and tear on muscles of the body. Creatinine levels in the blood can vary, and each laboratory has its own normal range. In many labs the normal range is 0.6 to 1.2 mg/dL. Higher levels may be a sign that the kidneys are not working properly. As kidney disease progresses, the level of creatinine in the blood increases.
Blood urea nitrogen (BUN). Urea nitrogen also is produced from the breakdown of food protein. A normal BUN level is between 7 and 20 mg/dL. As kidney function decreases, the BUN level increases.
A urinalysis may show protein or other abnormalities. An abnormal urinalysis may occur 6 months to 10 or more years before symptoms of kidney failure appear. Some urine tests require only a few ounces of urine, but others require collection of all urine produced for a full 24 hours. A 24-hour urine test shows how much urine your kidneys produce in one day. The test is sometimes used to measure how much protein leaks from the kidney into the urine. However, protein leakage can also be accurately determined in a small sample of urine by measuring its protein and creatinine concentration.
A creatinine clearance test compares the creatinine in a 24-hour sample of urine to the creatinine level in the blood, to show how many milliliters of blood the kidneys are filtering out each minute (mL/min). The creatinine clearance can also be estimated accurately from the serum creatinine alone using well established prediction equations.
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